IMHO it’s a matter of politicians unwilling to face hard choices. Bush let the clock play out, and Obama certainly has no stomach for taking action. The Israelis don’t have the capability to stop it, even if they screwed up the courage, short of a nuclear attack. So Iran will have a bomb, if they don’t already. I predict they will not test one until they have a stockpile built up. They don’t need to, since they are sharing designs with NK.
This is more than an unwillingness to face tough choices. It is fact that they have toyed with Iran and NK for well over a decade on this issue.
I’m content to conclude they did not do something because they did not want to do something. They want NK & Iran to have the bomb. The question is why?